US2005/001302 describes a screw compressor for gaseous media, in particular refrigerants, comprising a compressor housing, two screw rotors which are arranged in screw rotor bores in the compressor housing, which are rotatingly drivable and interact to compress the medium, and a control slide arranged adjacent the screw rotors and movable in a direction of displacement for controlling the compression of the screw compressor. In order to solve the problem that the compression of the screw compressor can be controlled, but not precisely regulated, it is proposed that a scannable element, which is scannable with a measuring sensor so as to recognize a position of the control slide in the direction of displacement, is coupled with the control slide.
U.S. Pat. No. 4,913,634 describes a screw compressor in which a pair of screw rotors supported by bearings and accommodated in a casing acts to compress a gas and in which a slide valve disposed between an inner wall of the casing and the pair of screw rotors are capable of moving axially while maintaining a small gap between itself and the outer peripheries of the screw rotors, the rate of gas flow bypassed to an inlet port during compression being regulated by axially moving the slide valve. The screw compressor has a side cover in which a bearing for supporting the pair of screw rotors on the discharge side and a slide valve driving hydraulic means are incorporated and which are disposed on the discharge side of the screw rotors; a discharge axial port formed in the side cover; and at least one projection continuous with an opening edge of the discharge axial port and in contact with a semi-circular surface of the slide valve facing in the radial direction thereof, the projection acting to limit the radial movement of the slide valve while the slide valve is moving in contact with the top end of the projection
U.S. Pat. No. 4,281,975 describes a screw compressor which includes a contacting a male and female rotor with respect to which a slide valve is operable to control the discharge of the compressor and also regulate the pressure of the fluid pumped or compressed thereby, where the improvement comprises a limited number of different embodiments of anti-friction constructions for supporting the slide valve within the cylindrical bore provided therefore in the housing within which the male and female rotors operate, the wear upon the slide valve within the bore therefore is thereby minimized and the force required to move the slide valve is minimized.
U.S. Pat. No. 5,044,909 concerns a rotary compressor of a cooling or heat pump system, the inner volume relation should be related in a predetermined way to the pressure relation of the compressor for an optimal efficiency to be achieved. The built-in volume relation must therefore be variable to be adapted, for example, to full load and partial load. In order to achieve the highest efficiency with respect to loading requirements, a valve device has been developed, in which the discharge port is formed in such a way as to substantially correspond to the theoretically correct radial discharge port and in which a valve body adapted for the purpose has its line of action oriented towards the outlet plane. The valve body is arranged in such a way that in its fully inserted position in the outlet port the mantle wall will correspond to the mantle wall of the working space of the compressor, and will be adjacent to the rotors at a minimum amount of play by means of the end face having been provided with a pointed line surrounded by two concave surface. The outlet port in the mantle wall of the working space of the rotary compressor is delimited by an outlet plane of the compressor and by the screw lines in the mantle wall, which correspond to the cam surfaces of the rotors, which cam surfaces interact in the direction of the outlet plane of the compressor.
It is of the utmost importance for the valve body to move on a certain plane in order not to get into a wrong position relative to its correct position in relation to the outlet port and the outlet plane; for example, the valve body can be given, for example, a substantially circular-cylindrical cross-section, whereby a good guiding surface for the valve body is achieved.
In order for the valve body not to rotate during its movement up to the outlet port and back again, the valve body can be provided with guiding means, such as grooves. With the valve body in an inclined position relatively to the running rotors, the outlet port can be formed more or less in correspondence with the theoretical outlet port, and at the same time a fully closed sealing line between the rotor housing and the outlet plane is achieved, which also contributes towards making the rotary compressor easier to handle; for example, when the rotors are to be inspected, only the valve body has to be removed. Further, the inclined position implies forming the valve body with an oblique surface, which will move towards the outlet plane, whereby the valve body will be guided in its movement towards the outlet port and will finally be fixed in its fully inserted position.